There are no set rules and different institutions will act in different ways. We compile a shortlist and call references when we know we want to interview a candidate. If you are concerned then I would state in CV that you would like to be informed prior to a certain reference being called. I would see this as a candidate asking for discretion whilst being respectful to her current employers. Heads are fair likelier to give a negative reference if they are surprised by a reference request than if they have been pre-warned. Finally, most people believe that they are a good judge of character and Heads in particular are prone to thinking this. Yet In all he schools that I've worked in there have been a few colleagues that I can't believe have been employed. My point is that most Heads will have wrongly doubted negative references, been lied to at interview and wrongly trusted positive references that they should have seen through. To conclude, I would remind the Head that they had agreed to be a referee if you have added them as a referee or contact the prospective employer and say that you would prefer if they could contact you before seeking references.